The present invention relates to a modulator mechanism for a rotary dobby, the input side of said modulator mechanism being connected to a drive means which is capable of rotating and the output side thereof providing an output which is applied to a main shaft controlling heald shafts and which is temporally modulated relative to the rotary movement of the drive means in such a way that a delay of the movements of the heald shafts at their maximum displacement positions is caused.
Such modulator mechanisms for rotary dobbies are known e.g. from the U.S. Pat. 5,107,901. Rotary dobbies including a drive means, which rotates at an essentially constant angular velocity, are provided with a modulator mechanism so as to drive a main shaft continuously in a certain direction at a modulated rotary speed. The rotary motion of the main shaft is converted into a linear movement of a heald frame via a crank and a connecting rod articulated on said crank, the movement of the heald frame at its two maximum displacement positions being delayed due to the modulated rotary motion of the main shaft. At each of these maximum displacement positions, a shed for weft insertion is formed, a longer shed rest being achieved due to the irregular movement of the main shaft.
In the above-mentioned prior art, a cycloidal mechanism and a disc cam mechanism with stationary cam discs and with a rotating roller lever are shown, which convert a rotary motion taking place at a constant angular velocity into a rotary motion taking place in a uniform direction at a variable angular velocity and points of rest for producing periods of rest at the dead centers. Due to the maintenance of a rotary motion in a uniform direction at the output of the modulator mechanism, the periods of rest which can be achieved at the dead points as well as the shed rest angles which can be obtained are limited by the structural design of the modulator mechanism in question.
For various weaving techniques, e.g. for the production of fabrics for technical use, or for fabrics having a particularly large width, large shed rest angles and long periods of rest at the dead centers of the crank arm of a rotary dobby are necessary so as to open the shed for a period of time which is sufficiently long for weft insertion. The large shed rest angles required in this connection cannot be achieved by the modulator mechanisms known according to the prior art.